If you own or are thinking about buying a house or some undeveloped land, it's good to learn about the nature of the soil. This will provide guidance about the most suitable garden plants, flowers and trees. You will also learn about drainage and potential stability problems for existing structures or those planned for the site.

My family once lived near San Marcos in a house built on Comfort-Rock outcrop, the formal name for a film of black clay over limestone. This soil is so thin that the U.S. Department of Agriculture classifies it as completely unsuitable for growing crops.

We quickly learned that it was impossible to plant a garden in a half-inch layer of black clay. We had to use a steel wrecking bar to pry hundreds of heavy limestone slabs from the earth and replace them with three dump-truck loads of top soil. The garden was outlined with stones we excavated.

Our artificial garden was very productive, but so was the undisturbed land nearby. Somehow Hill Country junipers, live oaks, Spanish oaks, mountain laurels, agaritas and bluebonnets managed to thrive in what little soil they found.

We now live over Branyon clay, a blackland soil that extends from Bexar County to North Central Texas. This soil is well suited for agriculture. But it has a well-deserved reputation for swelling when wet and contracting when dry. Paved roads and concrete slabs must be carefully designed to survive these conditions.

Soil classifications refer only to the uppermost layer of dirt, and many surprises can lie deeper below. A major sewer and road project near our place dramatically illustrates this.

While most of the project is through what is classified as Branyon clay at the surface, trenches up to 26 feet deep reveal many variations below. At some sites the clay is 12 feet or more thick. Elsewhere, less than a foot of clay interwoven by the roots of trees overlies limestone gravel and rocks. There is even chalk in places. Thick limestone layers are just below the surface in a few areas.

You can find general information about the soil where you live by entering “soil” and the name of your county into an Internet search engine. Or visit pssat.org/TexasSoils.htm.

Texas A&M and private labs offer soil-testing services for a fee. The Texas A&M soil-testing service is online at soiltesting.tamu.edu.

Forrest Mims, an amateur scientist whose research has appeared in leading scientific journals, was named one of the "50 Best Brains in Science” by Discover Magazine. His science is featured at www.forrestmims.